Regarding a technique of searching radio idler frequencies (white spaces), NPTL 1 discloses a technique related to a power detection circuit. FIG. 22 is a block diagram for describing a power detection circuit according to a related art. As shown in FIG. 22, the power detection circuit according to the related art includes a mixer 101, a variable frequency oscillator 102, a low pass filter 103, and an energy detection circuit 104.
The variable frequency oscillator 102 outputs a carrier wave 106 having a frequency equal to a reception frequency. The mixer 101 multiples the carrier wave 106 having a frequency equal to a reception frequency output from the variable frequency oscillator 102 by a reception signal 105, and outputs an in-phase component to the low pass filter 103. The low pass filter 103 removes out-of-band signals from the signal output from the mixer 101. The energy detection circuit 104 detects power in the bandwidth of the low pass filter 103.
Next, with reference to FIG. 23, an operation of the power detection circuit shown in FIG. 22 will be described. First, the frequency of the variable frequency oscillator 102 is set to an LO (local oscillator) frequency (local oscillator frequency: fLO) which is equal to an RF (radio) frequency (fRF) signal which is desired to be detected. This system is generally called a zero IF system or a direct conversion system. Then, as shown in FIG. 23, the mixer 101 converts the frequency of the RF frequency signal to a frequency around zero frequency (hereinafter referred to as a frequency around DC). Then, the low pass filter 103 removes the out-of-band signals, and the energy detection circuit 104 detects power in the filter bandwidth. Ideally, this detection power is equal to the power around the filter bandwidth of the RF frequency (fRF). Further, by sweeping the LO frequency (fLO) by changing the frequency of the variable frequency oscillator 102, it is possible to detect power of the RF frequency (fRF) for a wide bandwidth.